Owner's manual for my 19RD states Group 27 to be the design size. However 2 Group 27's will not fit the battery tray on the tongue. If you mostly use hookups I am sure a group 24 would be fine and you can probably fit 2 of them in the tray if you need extra capacity for dry camping.

I think this issue was brought to light a couple years ago by another member - most of the time the dealerships are installing Group 24 batteries while in fact the manual might state a Group 27 should be in there. If this is the case you could, in theory, get the dealership to upgrade it for you.